Molecular identity and functional properties of a novel T-type Ca2+ channel cloned from the sensory epithelia of the mouse inner ear

Multiple Calcium Channel Types with Unique Expression Patterns Mediate Retinal Signaling at Bipolar Cell Ribbon Synapses

Retinal bipolar cells (BCs) compose the canonical vertical excitatory pathway that conveys photoreceptor output to inner retinal neurons. Although synaptic transmission from BC terminals is thought to rely almost exclusively on Ca2+ influx through voltage-gated Ca2+ (CaV) channels mediating L-type currents, the molecular identity of CaV channels in BCs is uncertain. Therefore, we combined molecular and functional analyses to determine the expression profiles of CaV α1, β, and α2δ subunits in mouse rod bipolar (RB) cells, BCs from which the dynamics of synaptic transmission are relatively well-characterized. We found significant heterogeneity in CaV subunit expression within the RB population from mice of either sex, and significantly, we discovered that transmission from RB synapses was mediated by Ca2+ influx through P/Q-type (CaV2.1) and N-type (CaV2.2) conductances as well as the previously-described L-type (CaV1) and T-type (CaV3) conductances. Furthermore, we found both CaV1.3 and CaV1.4 proteins located near presynaptic ribbon-type active zones in RB axon terminals, indicating that the L-type conductance is mediated by multiple CaV1 subtypes. Similarly, CaV3 α1, β, and α2δ subunits also appear to obey a "multisubtype" rule, i.e., we observed a combination of multiple subtypes, rather than a single subtype as previously thought, for each CaV subunit in individual cells.SIGNIFICANCE STATEMENT Bipolar cells (BCs) transmit photoreceptor output to inner retinal neurons. Although synaptic transmission from BC terminals is thought to rely almost exclusively on Ca2+ influx through L-type voltage-gated Ca2+ (CaV) channels, the molecular identity of CaV channels in BCs is uncertain. Here, we report unexpectedly high molecular diversity of CaV subunits in BCs. Transmission from rod bipolar (RB) cell synapses can be mediated by Ca2+ influx through P/Q-type (CaV2.1) and N-type (CaV2.2) conductances as well as the previously-described L-type (CaV1) and T-type (CaV3) conductances. Furthermore, CaV1, CaV3, β, and α2δ subunits appear to obey a "multisubtype" rule, i.e., a combination of multiple subtypes for each subunit in individual cells, rather than a single subtype as previously thought.

Epilepsy and Hearing Loss in a Patient with a Rare Heterozygous Variant in the <i>CACNA1H</i> Gene

The calcium voltage-gated channel subunit alpha 1 H (<i>CACNA1H</i>) is a gene present in eukaryotic cells located on chromosome 16 that encodes the T-type calcium channels, which are important for calcium influx and depolarization of cells. Pathogenic variants in <i>CACNA1H</i> cause autosomal dominant susceptibility to idiopathic generalized epilepsy 6 (OMIM: 611942), which is a broad term that encompasses several common seizure phenotypes. In this article, we reported a Saudi female with a heterozygous variant in the <i>CACNA1H</i> gene (OMIM: 607904) who had epilepsy and hearing loss. This is the first case to report the association of epilepsy and hearing loss with a variant in <i>CACNA1H</i>. We believe that variant may be the reason for developing both epilepsy and sensorineural hearing loss. Further studies are needed to identify the role of <i>CACNA1H</i> in the physiology of the ear to allow for a better understanding of the effects of mutations. In children who present with early childhood hearing loss, genetic studies in highly selected cases including those with a strong family history of hearing loss and epilepsy may help detect a <i>CACNA1H</i> variant early and help with the early screening and diagnosis of other associated disorders including subclinical epilepsy.

Synaptic transmission at the vestibular hair cells of amniotes

A harmonized interplay between the central nervous system and the five peripheral end organs is how the vestibular system helps organisms feel a sense of balance and motion in three-dimensional space. The receptor cells of this system, much like their cochlear equivalents, are the specialized hair cells. However, research over the years has shown that the vestibular endorgans and hair cells evolved very differently from their cochlear counterparts. The structurally unique calyceal synapse, which appeared much later in the evolutionary time scale, and continues to intrigue researchers, is now known to support several forms of synaptic neurotransmission. The conventional quantal transmission is believed to employ the ribbon structures, which carry several tethered vesicles filled with neurotransmitters. However, the field of vestibular hair cell synaptic molecular anatomy is still at a nascent stage and needs further work. In this review, we will touch upon the basic structure and function of the peripheral vestibular system, with the focus on the various modes of neurotransmission at the type I vestibular hair cells. We will also shed light on the current knowledge about the molecular anatomy of the vestibular hair cell synapses and vestibular synaptopathy.

Downregulation of Cav3.1 T-type Calcium Channel Expression in Age-related Hearing Loss Model

OBJECTIVE

Age-related hearing loss (AHL), characterized by degeneration of cochlea structures, is the most common sensory disorder among the elderly worldwide. The calcium channel is considered to contribute to normal hearing. However, the role of the T-type voltage-activated calcium channel, Cav3.1, remains unclear in AHL. Here, we investigate the age-related change of Cav3.1 expression in the cochlea and D-gal-induced senescent HEI-OC1 cells.

METHODS

Cochleae from C57BL/6 mice at 2 months and 12 months of age were assessed. Senescence in House Ear Institute-Organ of Corti 1 (HEI-OC1) cells was induced by D-gal treatment. The immunofluorescence technique was employed to investigate the distribution of Cav3.1 in vivo and in vitro. Quantitative assessment was achieved by Western blotting and real-time PCR.

RESULTS

In comparison with 2-month-old animals, 12-month old C57BL/6 mice exhibited great loss of hair cells and elevated auditory brainstem threshold. The Cav3.1 was located in hair cells, spiral ganglion cells, lateral walls, and the expression of Cav3.1 protein and mRNA decreased in the aged cochleae. D-gal-induced senescence assay confirmed the down-regulation of Cav3.1 expression in senescent HEI-OC1 cells.

CONCLUSION

Our results show that age-related down-regulated expression of Cav3.1 in the cochleae is associated with AHL and may contribute to the pathogenesis of AHL.

CACHD1-deficient mice exhibit hearing and balance deficits associated with a disruption of calcium homeostasis in the inner ear

CACHD1 recently was shown to be an α2δ-like subunit that can modulate the activity of some types of voltage-gated calcium channels, including the low-voltage activated, T-type Ca_V3 channels. CACHD1 is widely expressed in the central nervous system but its biological functions and relationship to disease states are unknown. Here, we report that mice with deleterious Cachd1 mutations are hearing impaired and have balance defects, demonstrating that CACHD1 is functionally important in the peripheral auditory and vestibular organs of the inner ear. The vestibular dysfunction of Cachd1 mutant mice, exhibited by leaning and head tilting behaviors, is related to a deficiency of calcium carbonate crystals (otoconia) in the saccule and utricle. The auditory dysfunction, shown by ABR threshold elevations and reduced DPOAEs, is associated with reduced endocochlear potentials and increased endolymph calcium concentrations. Paint-fills of mutant inner ears from prenatal and newborn mice revealed dilation of the membranous labyrinth caused by an enlarged volume of endolymph. These pathologies all can be related to a disturbance of calcium homeostasis in the endolymph of the inner ear, presumably caused by the loss of CACHD1 regulatory effects on voltage-gated calcium channel activity. Cachd1 expression in the cochlea appears stronger in late embryonic stages than in adults, suggesting an early role in establishing endolymph calcium concentrations. Our findings provide new insights into CACHD1 function and suggest the involvement of voltage-gated calcium channels in endolymph homeostasis, essential for normal auditory and vestibular function.

Functional implications of Cav 2.3 R-type voltage-gated calcium channels in the murine auditory system - novel vistas from brainstem-evoked response audiometry

Voltage-gated Ca²⁺ channels (VGCCs) are considered to play a key role in auditory perception and information processing within the murine inner ear and brainstem. In the past, Ca_v 1.3 L-type VGCCs gathered most attention as their ablation causes congenital deafness. However, isolated patch-clamp investigation and localization studies repetitively suggested that Ca_v 2.3 R-type VGCCs are also expressed in the cochlea and further components of the ascending auditory tract, pointing to a potential functional role of Ca_v 2.3 in hearing physiology. Thus, we performed auditory profiling of Ca_v 2.3^+/+ controls, heterozygous Ca_v 2.3^+/- mice and Ca_v 2.3 null mutants (Ca_v 2.3^-/- ) using brainstem-evoked response audiometry. Interestingly, click-evoked auditory brainstem responses (ABRs) revealed increased hearing thresholds in Ca_v 2.3^+/- mice from both genders, whereas no alterations were observed in Ca_v 2.3^-/- mice. Similar observations were made for tone burst-related ABRs in both genders. However, Ca_v 2.3 ablation seemed to prevent mutant mice from total hearing loss particularly in the higher frequency range (36-42 kHz). Amplitude growth function analysis revealed, i.a., significant reduction in ABR wave W_I and W_III amplitude in mutant animals. In addition, alterations in W_I -W_IV interwave interval were observed in female Ca_v 2.3^+/- mice whereas absolute latencies remained unchanged. In summary, our results demonstrate that Ca_v 2.3 VGCCs are mandatory for physiological auditory information processing in the ascending auditory tract.

Genetic T-type calcium channelopathies

T-type channels are low-voltage-activated calcium channels that contribute to a variety of cellular and physiological functions, including neuronal excitability, hormone and neurotransmitter release as well as developmental aspects. Several human conditions including epilepsy, autism spectrum disorders, schizophrenia, motor neuron disorders and aldosteronism have been traced to variations in genes encoding T-type channels. In this short review, we present the genetics of T-type channels with an emphasis on structure-function relationships and associated channelopathies.

Cav3.2 T-Type Calcium Channels Are Physiologically Mandatory for the Auditory System

Voltage-gated Ca²⁺ channels (VGCCs) play key roles in auditory perception and information processing within the inner ear and brainstem. Pharmacological inhibition of low voltage-activated (LVA) T-type Ca²⁺ channels is related to both age- and noise induced hearing loss in experimental animals and may represent a promising approach to the treatment of auditory impairment of various etiologies. Within the LVA Ca²⁺ channel subgroup, Ca_v3.2 is the most prominently expressed T-type channel entity in the cochlea and auditory brainstem. Thus, we performed a complete gender specific click and tone burst based auditory brainstem response (ABR) analysis of Ca_v3.2^+/- and Ca_v3.2^-/- mice, including i.a. temporal progression in hearing loss, amplitude growth function and wave latency analysis as well as a cochlear qPCR based evaluation of other VGCCs transcripts. Our results, based on a self-programmed automated wavelet approach, demonstrate that both heterozygous and Ca_v3.2 null mutant mice exhibit age-dependent increases in hearing thresholds at 5 months of age. In addition, complex alterations in W_I-IV amplitudes and latencies were detected that were not attributable to alterations in the expression of other VGCCs in the auditory tract. Our results clearly demonstrate the important physiological role of Ca_v3.2 VGCCs in the spatiotemporal organization of auditory processing in young adult mice and suggest potential pharmacological targets for interventions in the future.

Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear

Calcium influx through voltage-gated Ca (Ca_V) channels is the first step in synaptic transmission. This review concerns Ca_V channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the Ca_V channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of Ca_V channels at presynaptic, ribbon-type active zones, because the spatial relationship between Ca_V channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, Ca_V1.3, and Ca_V1.4 channels or their protein modulatory elements.

Confirming a Role for α9nAChRs and SK Potassium Channels in Type II Hair Cells of the Turtle Posterior Crista

In turtle posterior cristae, cholinergic vestibular efferent neurons (VENs) synapse on type II hair cells, bouton afferents innervating type II hair cells, and afferent calyces innervating type I hair cells. Electrical stimulation of VENs releases acetylcholine (ACh) at these synapses to exert diverse effects on afferent background discharge including rapid inhibition of bouton afferents and excitation of calyx-bearing afferents. Efferent-mediated inhibition is most pronounced in bouton afferents innervating type II hair cells near the torus, but becomes progressively smaller and briefer when moving longitudinally through the crista toward afferents innervating the planum. Sharp-electrode recordings have inferred that efferent-mediated inhibition of bouton afferents requires the sequential activation of alpha9-containing nicotinic ACh receptors (α9^*nAChRs) and small-conductance, calcium-dependent potassium channels (SK) in type II hair cells. Gradations in the strength of efferent-mediated inhibition across the crista likely reflect variations in α9^*nAChRs and/or SK activation in type II hair cells from those different regions. However, in turtle cristae, neither inference has been confirmed with direct recordings from type II hair cells. To address these gaps, we performed whole-cell, patch-clamp recordings from type II hair cells within a split-epithelial preparation of the turtle posterior crista. Here, we can easily visualize and record hair cells while maintaining their native location within the neuroepithelium. Consistent with α9^*nAChR/SK activation, ACh-sensitive currents in type II hair cells were inward at hyperpolarizing potentials but reversed near −90 mV to produce outward currents that typically peaked around −20 mV. ACh-sensitive currents were largest in torus hair cells but absent from hair cells near the planum. In current clamp recordings under zero-current conditions, ACh robustly hyperpolarized type II hair cells. ACh-sensitive responses were reversibly blocked by the α9nAChR antagonists ICS, strychnine, and methyllycaconitine as well as the SK antagonists apamin and UCL1684. Intact efferent terminals in the split-epithelial preparation spontaneously released ACh that also activated α9^*nAChRs/SK in type II hair cells. These release events were accelerated with high-potassium external solution and all events were blocked by strychnine, ICS, methyllycaconitine, and apamin. These findings provide direct evidence that activation of α9^*nAChR/SK in turtle type II hair cells underlies efferent-mediated inhibition of bouton afferents.

Otoprotective effects of ethosuximide in NOD/LtJ mice with age-related hearing loss

Despite long-term efforts to elucidate the mechanisms responsible for age-related hearing loss (AHL), there is currently no available treatment strategy able to provide a cure. Apoptotic cell death, including that of hair cells and spiral ganglion neurons (SGNs) in the cochlea has been proposed to be the classic theory behind the development of AHL. As calcium signaling plays key roles in signal transduction in apoptosis, in this study, we selected ethosuximide, which is able to block T-type calcium (Ca²⁺ion) channels, suppressing Ca²⁺. We hypothesized that the apoptotic pathway may be blocked through the inhibition of T-type Ca²⁺ channels in cochlear cells in NOD/LtJ mice. NOD/LtJ mice were divided into 2 groups as follows: the ethosuximide-treated and untreated (control) groups. Ethosuximide was administered by intraperitoneal injection every other day from post-natal day seven (P7) until the mice were 8 weeks of age. Following treatment, auditory-evoked brainstem response (ABR) thresholds and distortion product oto-acoustic emission (DPOAE) of the mice in the 2 groups were measured at different time points. Morphometric analysis and the expression of genes involved in the T-type Ca²⁺-mediated apoptotic pathway were monitored. The ABR and DPOAE results revealed that the NOD/LtJ mice exhibited early-onset and rapidly progressive AHL. A histological examination revealed that hair cell degeneration coincided with the progression of hearing loss. Hair cell and SGN was were significantly lower and auditory function was significantly improved in the ethosuximide-treated group compared to the untreated group. Our data thus indicate that ethosuximide prevents the degeneration of cochlear cells by regulating the expression of genes in apoptotic pathways. Our findings suggest that activating the T-type Ca²⁺ channel and downstream genes may be key pathological mechanisms responsible for AHL in NOD/LtJ mice.

AMPA receptor-mediated rapid EPSCs in vestibular calyx afferents

In the vestibular periphery neurotransmission between hair cells and primary afferent nerves occurs via specialized ribbon synapses. Type I vestibular hair cells (HCIs) make synaptic contacts with calyx terminals, which enclose most of the HCI basolateral surface. To probe synaptic transmission, whole cell patch-clamp recordings were made from calyx afferent terminals isolated together with their mature HCIs from gerbil crista. Neurotransmitter release was measured as excitatory postsynaptic currents (EPSCs) in voltage clamp. Spontaneous EPSCs were classified as simple or complex. Simple events exhibited a rapid rise time and a fast monoexponential decay (time constant < 1 ms). The remaining events, constituting ~40% of EPSCs, showed more complex characteristics. Extracellular Sr²⁺ greatly increased EPSC frequency, and EPSCs were blocked by the AMPA receptor blocker NBQX. The role of presynaptic Ca²⁺ channels was assessed by application of the L-type Ca²⁺ channel blocker nifedipine (20 µM), which reduced EPSC frequency. In contrast, the L-type Ca²⁺ channel opener BAY K 8644 increased EPSC frequency. Cyclothiazide increased the decay time constant of averaged simple EPSCs by approximately twofold. The low-affinity AMPA receptor antagonist γ-d-glutamylglycine (2 mM) reduced the proportion of simple EPSCs relative to complex events, indicating glutamate accumulation in the restricted cleft between HCI and calyx. In crista slices EPSC frequency was greater in central compared with peripheral calyces, which may be due to greater numbers of presynaptic ribbons in central hair cells. Our data support a role for L-type Ca²⁺ channels in spontaneous release and demonstrate regional variations in AMPA-mediated quantal transmission at the calyx synapse.NEW & NOTEWORTHY In vestibular calyx terminals of mature cristae we find that the majority of excitatory postsynaptic currents (EPSCs) are rapid monophasic events mediated by AMPA receptors. Spontaneous EPSCs are reduced by an L-type Ca²⁺ channel blocker and notably enhanced in extracellular Sr²⁺ EPSC frequency is greater in central areas of the crista compared with peripheral areas and may be associated with more numerous presynaptic ribbons in central hair cells.

The Physiology, Pathology, and Pharmacology of Voltage-Gated Calcium Channels and Their Future Therapeutic Potential

Voltage-gated calcium channels are required for many key functions in the body. In this review, the different subtypes of voltage-gated calcium channels are described and their physiologic roles and pharmacology are outlined. We describe the current uses of drugs interacting with the different calcium channel subtypes and subunits, as well as specific areas in which there is strong potential for future drug development. Current therapeutic agents include drugs targeting L-type Ca(V)1.2 calcium channels, particularly 1,4-dihydropyridines, which are widely used in the treatment of hypertension. T-type (Ca(V)3) channels are a target of ethosuximide, widely used in absence epilepsy. The auxiliary subunit α2δ-1 is the therapeutic target of the gabapentinoid drugs, which are of value in certain epilepsies and chronic neuropathic pain. The limited use of intrathecal ziconotide, a peptide blocker of N-type (Ca(V)2.2) calcium channels, as a treatment of intractable pain, gives an indication that these channels represent excellent drug targets for various pain conditions. We describe how selectivity for different subtypes of calcium channels (e.g., Ca(V)1.2 and Ca(V)1.3 L-type channels) may be achieved in the future by exploiting differences between channel isoforms in terms of sequence and biophysical properties, variation in splicing in different target tissues, and differences in the properties of the target tissues themselves in terms of membrane potential or firing frequency. Thus, use-dependent blockers of the different isoforms could selectively block calcium channels in particular pathologies, such as nociceptive neurons in pain states or in epileptic brain circuits. Of important future potential are selective Ca(V)1.3 blockers for neuropsychiatric diseases, neuroprotection in Parkinson's disease, and resistant hypertension. In addition, selective or nonselective T-type channel blockers are considered potential therapeutic targets in epilepsy, pain, obesity, sleep, and anxiety. Use-dependent N-type calcium channel blockers are likely to be of therapeutic use in chronic pain conditions. Thus, more selective calcium channel blockers hold promise for therapeutic intervention.

Exocytotic machineries of vestibular type I and cochlear ribbon synapses display similar intrinsic otoferlin-dependent Ca2+ sensitivity but a different coupling to Ca2+ channels

The hair cell ribbon synapses of the mammalian auditory and vestibular systems differ greatly in their anatomical organization and firing properties. Notably, vestibular Type I hair cells (VHC-I) are surrounded by a single calyx-type afferent terminal that receives input from several ribbons, whereas cochlear inner hair cells (IHCs) are contacted by several individual afferent boutons, each facing a single ribbon. The specificity of the presynaptic molecular mechanisms regulating transmitter release at these different sensory ribbon synapses is not well understood. Here, we found that exocytosis during voltage activation of Ca(2+) channels displayed higher Ca(2+) sensitivity, 10 mV more negative half-maximum activation, and a smaller dynamic range in VHC-I than in IHCs. VHC-I had a larger number of Ca(2+) channels per ribbon (158 vs 110 in IHCs), but their Ca(2+) current density was twofold smaller because of a smaller open probability and unitary conductance. Using confocal and stimulated emission depletion immunofluorescence microscopy, we showed that VHC-I had fewer synaptic ribbons (7 vs 17 in IHCs) to which Cav1.3 channels are more tightly organized than in IHCs. Gradual intracellular Ca(2+) uncaging experiments revealed that exocytosis had a similar intrinsic Ca(2+) sensitivity in both VHC-I and IHCs (KD of 3.3 ± 0.6 μM and 4.0 ± 0.7 μM, respectively). In otoferlin-deficient mice, exocytosis was largely reduced in VHC-I and IHCs. We conclude that VHC-I and IHCs use a similar micromolar-sensitive otoferlin Ca(2+) sensor and that their sensory encoding specificity is essentially determined by a different functional organization of Ca(2+) channels at their synaptic ribbons.

The temporal characteristics of Ca2+ entry through L-type and T-type Ca2+ channels shape exocytosis efficiency in chick auditory hair cells during development

During development, synaptic exocytosis by cochlear hair cells is first initiated by patterned spontaneous Ca(2+) spikes and, at the onset of hearing, by sound-driven graded depolarizing potentials. The molecular reorganization occurring in the hair cell synaptic machinery during this developmental transition still remains elusive. We characterized the changes in biophysical properties of voltage-gated Ca(2+) currents and exocytosis in developing auditory hair cells of a precocial animal, the domestic chick. We found that immature chick hair cells (embryonic days 10-12) use two types of Ca(2+) currents to control exocytosis: low-voltage-activating, rapidly inactivating (mibefradil sensitive) T-type Ca(2+) currents and high-voltage-activating, noninactivating (nifedipine sensitive) L-type currents. Exocytosis evoked by T-type Ca(2+) current displayed a fast release component (RRP) but lacked the slow sustained release component (SRP), suggesting an inefficient recruitment of distant synaptic vesicles by this transient Ca(2+) current. With maturation, the participation of L-type Ca(2+) currents to exocytosis largely increased, inducing a highly Ca(2+) efficient recruitment of an RRP and an SRP component. Notably, L-type-driven exocytosis in immature hair cells displayed higher Ca(2+) efficiency when triggered by prerecorded native action potentials than by voltage steps, whereas similar efficiency for both protocols was found in mature hair cells. This difference likely reflects a tighter coupling between release sites and Ca(2+) channels in mature hair cells. Overall, our results suggest that the temporal characteristics of Ca(2+) entry through T-type and L-type Ca(2+) channels greatly influence synaptic release by hair cells during cochlear development.

Postnatal expression of an apamin-sensitive k(ca) current in vestibular calyx terminals

Afferent innervation patterns in the vestibular periphery are complex, and vestibular afferents show a large variation in their regularity of firing. Calyx fibers terminate on type I vestibular hair cells and have firing characteristics distinct from the bouton fibers that innervate type II hair cells. Whole-cell patch clamp was used to investigate ionic currents that could influence firing patterns in calyx terminals. Underlying K(Ca) conductances have been described in vestibular ganglion cells, but their presence in afferent terminals has not been investigated previously. Apamin, a selective blocker of SK-type calcium-activated K(+) channels, was tested on calyx afferent terminals isolated from gerbil semicircular canals during postnatal days 1-50. Lowering extracellular calcium or application of apamin (20-500 nM) reduced slowly activating outward currents in voltage clamp. Apamin also reduced the action potential afterhyperpolarization (AHP) in whole-cell current clamp, but only after the first two postnatal weeks. K(+) channel expression increased during the first postnatal month, and SK channels were found to contribute to the AHP, which may in turn influence discharge regularity in calyx vestibular afferents.

Anti-epileptic drugs delay age-related loss of spiral ganglion neurons via T-type calcium channel

Loss of spiral ganglion neurons is a major cause of age-related hearing loss (presbycusis). Despite being the third most prevalent condition afflicting elderly persons, there are no known medications to prevent presbycusis. Because calcium signaling has long been implicated in age-related neuronal death, we investigated T-type calcium channels. This family is comprised of three members (Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3), based on their respective main pore-forming alpha subunits: α1G, α1H, and α1I. In the present study, we report a significant delay of age-related loss of cochlear function and preservation of spiral ganglion neurons in α1H null and heterozygous mice, clearly demonstrating an important role for Ca(v)3.2 in age-related neuronal loss. Furthermore, we show that anticonvulsant drugs from a family of T-type calcium channel blockers can significantly preserve spiral ganglion neurons during aging. To our knowledge, this is the first report of drugs capable of diminishing age-related loss of spiral ganglion neurons.

Why do hair cells and spiral ganglion neurons in the cochlea die during aging?

Age-related decline of cochlear function is mainly due to the loss of hair cells and spiral ganglion neurons (SGNs). Recent findings clearly indicate that survival of these two cell types during aging depends on genetic and environmental interactions, and this relationship is seen at the systemic, tissue, cellular, and molecular levels. At cellular and molecular levels, age-related loss of hair cells and SGNs can occur independently, suggesting distinct mechanisms for the death of each during aging. This mechanistic independence is also observed in the loss of medial olivocochlear efferent innervation and outer hair cells during aging, pointing to a universal independent cellular mechanism for age-related neuronal death in the peripheral auditory system. While several molecular signaling pathways are implicated in the age-related loss of hair cells and SGNs, studies with the ability to locally modify gene expression in these cell types are needed to address whether these signaling pathways have direct effects on hair cells and SGNs during aging. Finally, the issue of whether age-related loss of these cells occurs via typical apoptotic pathways requires further examination. As new studies in the field of aging reshape the framework for exploring these underpinnings, understanding of the loss of hair cells and SGNs associated with age and the interventions that can treat and prevent these changes will result in dramatic benefits for an aging population.

Otoferlin is critical for a highly sensitive and linear calcium-dependent exocytosis at vestibular hair cell ribbon synapses

Otoferlin, a C2-domain-containing Ca(2+) binding protein, is required for synaptic exocytosis in auditory hair cells. However, its exact role remains essentially unknown. Intriguingly enough, no balance defect has been observed in otoferlin-deficient (Otof(-/-)) mice. Here, we show that the vestibular nerve compound action potentials evoked during transient linear acceleration ramps in Otof(-/-) mice display higher threshold, lower amplitude, and increased latency compared with wild-type mice. Using patch-clamp capacitance measurement in intact utricles, we show that type I and type II hair cells display a remarkable linear transfer function between Ca(2+) entry, flowing through voltage-activated Ca(2+) channels, and exocytosis. This linear Ca(2+) dependence was observed when changing the Ca(2+) channel open probability or the Ca(2+) flux per channel during various test potentials. In Otof(-/-) hair cells, exocytosis displays slower kinetics, reduced Ca(2+) sensitivity, and nonlinear Ca(2+) dependence, despite morphologically normal synapses and normal Ca(2+) currents. We conclude that otoferlin is essential for a high-affinity Ca(2+) sensor function that allows efficient and linear encoding of low-intensity stimuli at the vestibular hair cell synapse.